Radiation sensor element and method

ABSTRACT

This disclosure relates to a radiation sensor element comprising a semiconductor substrate, having a bulk refractive index; a front surface; a back surface, extending substantially along a base plane; and a plurality of pixel portions. Each pixel portion comprises a collection region on the back surface and a textured region on the front surface. The textured regions comprise high aspect ratio nanostructures, extending substantially along a thickness direction perpendicular to the base plane and forming an optical conversion layer, having an effective refractive index gradually changing towards the bulk refractive index to reduce reflection of light incident on said pixel portion from the front side of the semiconductor substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/614,884, filed Nov. 29, 2021, which is a national phase entry ofInternational Application No. PCT/FI2020/050352, filed May 26, 2020,which claims priority to Finnish Application No. 20195457, filed May 31,2019, which is incorporated herein by reference in their entirety.

FIELD OF TECHNOLOGY

This disclosure concerns radiation detectors. In particular, thisdisclosure concerns semiconductor pixel detectors.

BACKGROUND

Semiconductor pixel detectors are used widely in consumer electronics,for example, in cameras, as well as in a variety of industrial andscientific settings, for example, in photodetectors, X-ray detectors,and particle detectors.

In conventional detectors, cross talk, e.g., optical cross talk, betweenindividual pixels may pose considerable challenges. Typically, opticalcross talk has been reduced by depositing dielectric antireflectioncoating layers. However, dielectric antireflection coatings may possessreduced antireflective properties at high incidence angles. Moreover, incase of conventional semiconductor detectors comprising a scintillatorfor converting ionizing radiation to non-ionizing electromagneticradiation, coupling the scintillator to a substrate may be challenging.

In light of this, it may be desirable to develop new solutions relatedto semiconductor pixel detectors.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

According to a first aspect, a radiation sensor element is provided. Theradiation sensor element comprises a semiconductor substrate, havingbulk majority charge carriers of a first polarity, a bulk refractiveindex, a front surface, defining a front side of the semiconductorsubstrate, and a back surface, arranged opposite the front surface andextending substantially along a base plane.

The radiation sensor element comprises a plurality of pixel portions,each pixel portion of the plurality of pixel portions comprising acollection region on the back surface for collecting free chargecarriers of a second polarity opposite in sign to the first polarity.

Each pixel portion of the plurality of pixel portions comprises atextured region on the front surface, the textured region comprisinghigh aspect ratio nanostructures, extending substantially along athickness direction perpendicular to the base plane and forming anoptical conversion layer, having an effective refractive index graduallychanging towards the bulk refractive index to reduce reflection of lightemitted by a scintillator and incident on said pixel portion from thefront side of the semiconductor substrate.

The radiation sensor element comprises an intermediate portion betweentwo pixel portions of the plurality of pixel portions. The intermediateportion comprises an intermediate region on the front surface with aroot mean square (RMS) roughness lower than a RMS roughness of atextured region of either of the two pixel portions. The radiationsensor element comprises the scintillator coupled to the intermediateregion.

According to a second aspect, a method for fabricating a radiationsensor element comprising a plurality of pixel portions is provided. Themethod comprises providing a semiconductor substrate, having bulkmajority charge carriers of a first polarity, a bulk refractive index, afront surface, defining a front side of the semiconductor substrate, anda back surface, arranged opposite the front surface and extendingsubstantially along a base plane; for each pixel portion of theplurality of pixel portions, forming a collection region on the backsurface for collecting free charge carriers of a second polarityopposite in sign to the first polarity; for each pixel portion of theplurality of pixel portions, forming a textured region on the frontsurface, the textured region comprising high aspect rationanostructures, extending substantially along a thickness directionperpendicular to the base plane and forming an optical conversion layer,having an effective refractive index gradually changing towards the bulkrefractive index to reduce reflection of light emitted by a scintillatorand incident on said pixel portion from the front side of thesemiconductor substrate; forming an intermediate portion between twopixel portions of the plurality of pixel portions, the intermediateportion comprising an intermediate region on the front surface with aroot mean square (RMS) roughness lower than a RMS roughness of atextured region of either of the two pixel portions; and coupling thescintillator onto the intermediate region.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be better understood from the followingdetailed description read in light of the accompanying drawings,wherein:

FIG. 1 shows an isometric view of a radiation sensor element,

FIG. 2 depicts a partial cross-sectional view of the radiation sensorelement along cross-sectional plane II of FIG. 1 , and

FIG. 3 illustrates a method for fabricating a radiation sensor elementcomprising a plurality of pixel portions.

Unless specifically stated to the contrary, any drawing of theaforementioned drawings may be not drawn to scale such that any elementin said drawing may be drawn with inaccurate proportions with respect toother elements in said drawing in order to emphasize certain structuralaspects of the embodiment of said drawing.

Moreover, corresponding elements in the embodiments of any two drawingsof the aforementioned drawings may be disproportionate to each other insaid two drawings in order to emphasize certain structural aspects ofthe embodiments of said two drawings.

DETAILED DESCRIPTION

FIGS. 1 and 2 depict a radiation sensor element 100 according to anembodiment. In particular, FIG. 2 depicts a cross-sectional view of theradiation sensor element 100 along cross-sectional plane II of FIG. 1 .Since FIG. 2 depicts a cross section of the radiation sensor element100, FIG. 2 does not limit shapes of the embodiment of FIGS. 1 and 2 orany part(s) thereof in any direction forming an angle with thecross-sectional plane II of FIG. 1 . In other embodiments, a radiationsensor element may be identical, similar, or different to the radiationsensor element 100 of the embodiment of FIGS. 1 and 2 .

Herein, “radiation” is to be understood broadly, covering, for example,electromagnetic radiation and particle radiation. Radiation maygenerally correspond to ionizing radiation or non-ionizing radiation.

In this specification, “ionizing” radiation may refer to radiation withsufficient particle or photon energy to induce ionization in a medium.Ionizing radiation may comprise radiation with particle or photonenergies of at least 3.89 electron volts (eV), at least 10 eV, or atleast 33 eV, for example. On the other hand, “non-ionizing” radiationmay herein refer to radiation with insufficient particle or photonenergy to induce substantial ionization in a medium. Non-ionizingradiation may comprise radiation with particle or photon energies ofless than 33 eV, less than 10 eV, or less than 3.89 eV, for example.

Throughout this specification, a “radiation detector” may refer to acomplete, operable radiation detector. A radiation detector maygenerally comprise at least one radiation sensor. A radiation detectormay comprise also other elements, units, and/or structures.

In this disclosure, a “radiation sensor” may refer to an operable unit,module, or device configured to detect and/or measure radiation and toregister, indicate, and/or respond to said radiation.

Further, a “radiation sensor element” may refer to an element, which mayform, as such, a radiation sensor. Alternatively, a radiation sensorelement may be used as one element of a radiation sensor comprising alsoother elements and/or structures. A radiation sensor element maycomprise an active material, a physical property of which is utilized insaid radiation sensor element in order to register, indicate, and/orrespond to radiation incident on said active material. A radiationsensor element may correspond to an indirect-conversion radiation sensorelement or a direct-conversion radiation sensor element.

Throughout this disclosure, an “indirect-conversion radiation sensorelement” may refer to a radiation sensor element comprising ascintillator for converting ionizing radiation to non-ionizingelectromagnetic radiation and active material for detecting theelectromagnetic radiation emitted by the scintillator.

By contrast, a “direct-conversion radiation sensor element” may refer toa radiation sensor element not requiring the use of a scintillator toconvert ionizing radiation to non-ionizing electromagnetic radiation inorder to detect said ionizing radiation.

In this specification, a “scintillator” may refer to an element,comprising material that emits light when excited by ionizing radiation,such as particle radiation. Generally, luminescence of a scintillatormay result in emission of light to a large solid angle. Consequently,light from a scintillator may commonly reach a textured region at highincidence angles.

In the embodiment of FIGS. 1 and 2 , the radiation sensor element 100comprises a semiconductor substrate 101.

Herein, “semiconductor” may refer to material, such as silicon (Si) orgermanium (Ge), possessing an electrical conductivity intermediatebetween the conductivity of conductive materials, such as metals, andthe conductivity of insulating materials, such as many plastics andglasses. A semiconductor material may generally have a doping level,which may be adjusted in order to tune properties of said semiconductormaterial in a controllable manner.

Throughout this specification, a “substrate” may refer to a layer orother element or structure suitable for or configured to provide asurface whereon other layers or other elements or structures may becoupled, bonded, mounted, arranged, deposited, laminated, and/orfabricated. Consequently, a “semiconductor substrate” may refer to asubstrate comprising and/or formed of a semiconductor material, such asa semiconductor wafer or die.

The semiconductor substrate 101 of the embodiment of FIGS. 1 and 2 hasbulk majority charge carriers of a first polarity. In case of theembodiment of FIGS. 1 and 2 , the first polarity is negative. In otherembodiments, a semiconductor substrate may have bulk majority chargecarriers of a first polarity, which may be negative or positive.

Throughout this disclosure, a “charge carrier” may refer to a freelymoving particle or quasiparticle carrying electric charge in an elementor a part thereof. Generally, such element or a part thereof maycomprise one or more types of charge carriers. For example, asemiconductor may comprise electrons and holes as charge carriers.Consequently, a “majority charge carrier” may refer to a more common orabundant type of charge carrier in an element or a part thereof. Forexample, in an n-type semiconductor, electrons may act as majoritycharge carriers. Further, “bulk” may refer to a larger and/or greaterpart of an element. Additionally or alternatively, bulk may refer to aninner and/or central part of an element. As such, “bulk majority chargecarriers” may refer to majority charge carriers of an element in a bulkpart thereof.

The semiconductor substrate 101 of the embodiment of FIGS. 1 and 2 maybe formed of Si. In other embodiments, a semiconductor substrate maycomprise any suitable material(s), for example, Si and/or Ge.

The semiconductor substrate 101 of the embodiment of FIGS. 1 and 2 mayhave a bulk electrical resistivity of at least 100 ohm-meters (Ωm) atnormal temperature and pressure (NTP) conditions, corresponding to abulk majority charge carrier concentration less than or equal to about4×10¹¹ per cubic centimeter (cm⁻³). In other embodiments, asemiconductor substrate, which may or may not be formed of Si, may haveany suitable bulk majority charge carrier concentration, for example, abulk majority charge carrier concentration less than or equal to about1×10²⁰ cm⁻³, or 1×10¹⁶ cm⁻³, or 1×10¹² cm⁻³. The semiconductor substrate101 of the embodiment of FIGS. 1 and 2 has a bulk refractive index.

In this specification, “refractive index” of a medium may refer to aratio between speed of light in vacuum and a phase velocity of light insaid medium. Generally, the term “refractive index” may or may not referto a complex-valued refractive index. Further, a “bulk refractive index”of an element may refer to a refractive index of a bulk part of saidelement.

Herein, “light” may refer to electromagnetic radiation of any wavelengthwithin a range of relevant wavelengths. Such range of relevantwavelengths may or may not overlap or coincide with ultraviolet(wavelength from about 10 nanometers (nm) to about 400 nm), visible(wavelength from about 400 nm to about 700 nm), and/or infrared(wavelength from about 700 nm to about 1 millimeter (mm)) parts ofelectromagnetic spectrum.

The semiconductor substrate 101 of the embodiment of FIGS. 1 and 2 has afront surface 102. The front surface 102 of the semiconductor substrate101 defines a front side of the semiconductor substrate 101.

Throughout this disclosure, a “surface” may refer to a finite part of ageneralization of a plane, which may have a non-zero, possiblyposition-dependent, curvature and which may or may not be connected,path-connected, or simply connected. Additionally or alternatively, asurface may refer to a part of an outer boundary of a body or anelement. A surface may specifically refer to a part of an outer boundaryof a body or an element viewable from a particular viewing direction, ora part thereof.

The semiconductor substrate 101 of the embodiment of FIGS. 1 and 2 has aback surface 103. The back surface 103 of the semiconductor substrate101 is arranged opposite the front surface 102, and it extendssubstantially along a fictitious base plane 104.

Herein, a “base plane” may refer to a fictitious generalization of aplane, which may or may not have a non-zero, possibly position-dependentcurvature. As such, a base plane may or may not be planar.

The base plane 104 of the embodiment of FIGS. 1 and 2 is planar. Inother embodiments, a semiconductor substrate may be flat or curved,having a front surface, which extends substantially along a planar or acurved base plane, respectively.

In the embodiment of FIGS. 1 and 2 , a thickness direction is definedperpendicular to the base plane 104. The thickness direction extendsvertically in FIGS. 1 and 2 . In some embodiments, wherein a base planeis curved, a thickness direction may be position-dependent. In othersuch embodiments, a single position-independent thickness direction maybe defined.

In the embodiment of FIGS. 1 and 2 , the radiation sensor element 100comprises a plurality of pixel portions 110.

Throughout this disclosure, a “plurality” of elements or features mayrefer to a group of two or more, or three or more, etc., of saidelements or features, respectively. Further, a “portion” of a radiationsensor element may refer to a part of a radiation sensor element,extending from a region on a front surface of a semiconductor substrateto a region on a back surface of a semiconductor substrate. Herein, a“region” may refer to a part of a surface. Consequently, a “pixelportion” may refer to a portion of a radiation sensor element, which maybe utilized in said radiation sensor element in order to register,indicate, and/or respond to radiation incident specifically on saidpixel portion. Individual pixel portions of a plurality of pixelportions may or may not abut one another, i.e., they may or may notshare a common boundary with one another. Such common boundary mayextend, for example, within a semiconductor substrate.

In the embodiment of FIGS. 1 and 2 , individual pixel portions of theplurality of pixel portions 110 may possess identical or similarfeatures. As such, each pixel portion of the embodiment of FIGS. 1 and 2may belong to the plurality of pixel portions 110, and the radiationsensor element 100 may comprise only a single plurality of pixelportions 110. In other embodiments, a radiation sensor element maycomprise at least one (i.e., one or more, two or more, etc.) pluralityof pixel portions.

The plurality of pixel portions 110 of the embodiment of FIGS. 1 and 2forms a regular, rectangular two-dimensional array of pixel portions. Inother embodiments, a plurality of pixel portions may or may not form anarray of pixel portions, e.g., a one-dimensional or a two-dimensionalarray of pixel portions, which may or may not be regular and which mayhave any suitable symmetry properties.

Individual pixel portions of the plurality of pixel portions 110 of theembodiment of FIGS. 1 and 2 may have lateral sizes of 1 millimeter(mm)×1 mm parallel to the base plane 104. In other embodiments,individual pixel portions of a plurality of pixel portions may have anysuitable lateral shapes and sizes, for example, in case of substantiallycuboidal pixel portions, lateral sizes in a range from 5 micrometers(μm)×5 μm to mm×10 mm, or from 10 μm×10 μm to 1 mm×1 mm, or from 50μm×50 μm to 0.5 mm×0.5 mm.

In the embodiment of FIGS. 1 and 2 , each pixel portion of the pluralityof pixel portions 110 comprises a collection region 120 on the backsurface 103 of the semiconductor substrate 101.

Herein, a “collection region” may refer to a region, which may bearranged on a back surface of a semiconductor substrate with bulkmajority charge carriers of a first polarity, suitable for or configuredto collect free charge carriers of a second polarity opposite in sign tothe first polarity. Such free charge carriers may be collected, forexample, to an integrated or an external electrical read-out circuit.Specifically, a collection region of a pixel portion may be suitable foror configured to collect free charge carriers from said pixel portion.

As depicted schematically in FIG. 2 for two exemplary pixel portions111, 112, the collection regions 120 are defined by collection dopingwells 121. A collection region being defined by a collection doping wellmay promote separation of free charge carriers, which may increase aquantum efficiency of a radiation sensor element. In other embodiments,a collection region may be defined by any suitable means, for example,by a collection doping well, or an interface between a semiconductorsubstrate and a conductor pattern, such as a metallization pad or asolder bump, or a through-hole in a dielectric layer on a back surfaceof a semiconductor substrate.

In this disclosure, a “layer” may refer to a generally sheet-shapedelement arranged on a surface or a body. Additionally or alternatively,a layer may refer to one of a series of superimposed, overlaid, orstacked generally sheet-shaped elements. A layer may generally comprisea plurality of sublayers of different materials or materialcompositions. Some layers may be path-connected, whereas other layersmay be locally path-connected and disconnected.

Throughout this disclosure, a “dielectric” material may refer to amaterial, which may exhibit a low electrical conductivity. Additionallyor alternatively, dielectric material may be electrically polarizable.Generally, dielectric material may have any suitable relativepermittivity, for example, a relative permittivity of at least 2, atleast 3, at least 5, or at least 20. Consequently, a dielectric layermay refer to a layer comprising or formed of dielectric material.

Herein, existence of a “through-hole” in a layer may refer to a shape ofsaid layer being such that said layer comprises a discontinuity.Additionally or alternatively, a through-hole may refer to a hole in atopological (homeomorphism) sense.

The collection doping wells 121 of the embodiment of FIGS. 1 and 2 areformed in the semiconductor substrate 101. Such collection doping wellsmay generally be formed at least partly by a dopant implantation stepand/or a dopant diffusion step.

In the embodiment of FIGS. 1 and 2 , the radiation sensor element 100comprises a bulk contact region 122 on the back surface 103 of thesemiconductor substrate 101, as illustrated in FIG. 2 . Generally, suchbulk contact region on a back surface of a semiconductor substrate mayfacilitate coupling a scintillator onto a semiconductor substrate and/orenable arranging a plurality of radiation sensor elements in closeproximity to one another, when electrical connections for individualpixels are not necessary on one or more sides of a radiation sensorelement. In other embodiments, a radiation sensor element may compriseone or more bulk contact regions, which may or may not be arranged on aback surface of semiconductor substrate.

Throughout this specification, a “bulk contact region” may refer to aregion on a surface, such as a back surface, of a semiconductorsubstrate with bulk majority charge carriers of a first polarity,suitable for or configured to collect free charge carriers of the firstpolarity and/or to collect free charge carriers from a bulk part of saidsemiconductor substrate.

As depicted schematically in FIG. 2 , the bulk contact region 122 isdefined by bulk contact doping well 123.

A bulk contact region being defined by bulk contact doping well maypromote separation of free charge carriers, which may increase a quantumefficiency of a radiation sensor element. In other embodiments, a bulkcontact region may be defined by any suitable means, for example, by abulk contact doping well, or an interface between a semiconductorsubstrate and a conductor pattern, such as a metallization pad or asolder bump, or a through-hole in a dielectric layer on a semiconductorsubstrate.

Although not specifically depicted in either of FIGS. 1 and 2 , aradiation sensor element, such as the radiation sensor element 100 ofthe embodiment of FIGS. 1 and 2 , may generally comprise conductorpattern(s) electrically connected to a collection region and/or a bulkcontact region.

As depicted schematically in FIG. 2 for the two exemplary pixel portions111, 112, each pixel portion of the plurality of pixel portions 110comprises a textured region 130 on the front surface 102.

Herein, a “textured region” may refer to a non-smooth, patterned, and/ornanostructured region, which may be arranged on a front surface of asemiconductor substrate.

The textured regions 130 of the embodiment of FIGS. 1 and 2 comprisehigh aspect ratio nanostructures 135.

In this disclosure, “high aspect ratio nanostructures” may refer tonanostructures having their height, in a thickness direction, multipletimes their lateral dimensions. Such nanostructures may comprise, forexample, cylindrical pillars, conical pillars, or narrow pyramids.

The high aspect ratio nanostructures 135 of the embodiment of FIGS. 1and 2 extend substantially along the thickness direction. In otherembodiments, wherein a base plane is curved, high aspect rationanostructures may extend substantially along a position-dependentthickness direction or a single, position-independent thicknessdirection.

As depicted schematically in FIG. 2 for the two exemplary pixel portions111, 112, the high aspect ratio nanostructures 135 of the embodiment ofFIGS. 1 and 2 form optical conversion layers 136. The optical conversionlayers 136 have effective refractive indices n₁ ^(eff), n₂ ^(eff)gradually changing towards the bulk refractive index n^(B). This reducesreflection of light incident on individual pixel portions of theplurality of pixel portions 110 from the front side of the semiconductorsubstrate 101. Generally, such high aspect ratio nanostructures mayreduce optical cross talk between individual pixels of a radiationsensor element, especially in case a scintillator is to be arranged on afront side of at least one pixel portion of a radiation sensor element.Such reduction in optical cross talk may result from reduced reflectionswithin a radiation sensor element, for example, between a scintillatorand a semiconductor substrate.

Throughout this specification, an “optical conversion layer” may referto a layer, which may be indefinable based on continuous materialinterfaces, such as lateral interfaces, having an effective refractiveindex n^(eff) which gradually changes from an ambient refractive indexn^(i) towards a bulk refractive index n^(B) to reduce reflection oflight incident on a radiation sensor element from a front side thereof.For example, where the radiation sensor element is designed to be usedunder exposure to ambient air with a refractive index of about 1, theeffective refractive index may gradually change from said about 1 to thebulk refractive index n^(B).

Herein, an “effective refractive index” is an auxiliary definitionrelated to interaction of light with a nanostructured layer.Sub-wavelength features or features substantially in a range of relevantwavelengths may make light behave in such a nanostructured layerdifferently from a corresponding layer of the same material(s) in theabsence of such features. This different behavior may be described usingthe auxiliary term “effective refractive index”; light behaves in, andinteracts with, such a nanostructured layer as if the layer would bemade of a gradually changing bulk material having, at each level of theconversion layer, a refractive index equal to the effective refractiveindex n^(eff) at that level.

The high aspect ratio nanostructures 135 of the embodiment of FIGS. 1and 2 are arranged irregularly. Such irregular arrangement of highaspect ratio nanostructures may reduce a reflectance of a texturedregion. In other embodiments, high aspect ratio nanostructures may bearranged in any suitable arrangement(s), for example, irregularly orregularly.

In FIG. 2 , two effective refractive indices n₁ ^(eff), n₂ ^(eff) aredepicted for the two optical conversion layers 136. This indicates thatn₁ ^(eff), n₂ ^(eff) may be independent of one another. As such, n₁^(eff), n₂ ^(eff) may have identical, similar, or different values withone another at any given level of the optical conversion layers 136. Inother embodiments, any two effective refractive indices of any twooptical conversion layers may or may not be independent of one another.

The high aspect ratio nanostructures 135 of the embodiment of FIGS. 1and 2 are continuous and monolithic with the semiconductor substrate101. Furthermore, the high aspect ratio nanostructures 135 are formed ofa common material with the semiconductor substrate 101. In case of theembodiment of FIGS. 1 and 2 , such common material may be Si. Generally,high aspect ratio nanostructures formed of common material with and/orcontinuous with and/or monolithic with a semiconductor substrate maygenerally exhibit lowered recombination losses. In other embodiments,high aspect ratio nanostructures may or may not be formed of commonmaterial with and/or be continuous with and/or be monolithic with asemiconductor substrate. In said other embodiments, such common materialmay be any suitable material, for example, Si or Ge.

The high aspect ratio nanostructures 135 of the embodiment of FIGS. 1and 2 may be black silicon (b-Si) spikes. In other embodiments, highaspect ratio nanostructures may be b-Si spikes or any othernanostructures suitable for forming an optical conversion layer havingan effective refractive index gradually changing towards a bulkrefractive index.

Herein, “black silicon” may refer to a class of nanoscale surfaceformations on Si, producing an optical conversion layer having agradually changing effective refractive index. A b-Si surface maycomprise a plurality of needle- and/or spike-like surface formations.Individual surface formations of such a plurality of surface formationsmay be of varying sizes and/or arranged irregularly.

The high aspect ratio nanostructures 135 of the embodiment of FIGS. 1and 2 may have an average height in the thickness direction in a rangefrom 500 nm to 1500 nm. In other embodiments, high aspect rationanostructures may have any suitable average height in a thicknessdirection, for example, an average thickness in a range from 500 nm to1500 nm, or from about 600 nm to 1200 nm, or from 800 nm to 1000 nm.

The high aspect ratio nanostructures 135 of the embodiment of FIGS. 1and 2 , may have an average width in a lateral direction parallel to thebase plane 104 in a range from 50 nm to 500 nm. In other embodiments,high aspect ratio nanostructures may have any suitable average width ina thickness direction, for example, an average width in a range from 50nm to 500 nm, or from 100 nm to 400 nm, or from 200 nm to 300 nm.

As depicted schematically in FIG. 2 for the two exemplary pixel portions111, 112, the radiation sensor element 100 of the embodiment of FIGS. 1and 2 comprises dielectric material 150 conformally coating the highaspect ratio nanostructures 135. Such dielectric material may generallyreduce recombination losses of a radiation sensor element. In otherembodiments, a radiation sensor element may or may not comprise suchdielectric material.

Herein, material “conformally coating” high aspect ratio nanostructuresmay refer said material being formed in a shape, which follows said highaspect ratio nanostructures with a substantially uniform coatingthickness. Herein, a “substantially uniform coating thickness” may referto a relative standard deviation in coating thickness of less than 50%,or less than 25%, or less than 15%, and/or to a standard deviation inthickness of less than 20 nm, or less than 10 nm, or less than 5 nm.Generally, a coating thickness of material conformally coating highaspect ratio nanostructures may be measureable along surface normals ofsaid high aspect ratio nanostructures. Such coating thickness may be,for example, in a range from about 1 nm to 100 nm, or from about 2 nm to50 nm, or from about 3 nm to 30 nm, or from about 5 nm to about 20 nm.

The dielectric material 150 of the embodiment of FIGS. 1 and 2 forms alayer, extending continuously from the first pixel portion 111 depictedin FIG. 2 to the second pixel portion 112 depicted in FIG. 2 . In otherembodiments, dielectric material may be formed as one or more pieces,such as layers.

The dielectric material 150 of the embodiment may have a net charge ofthe second polarity. In case of the embodiment of FIGS. 1 and 2 , thesecond polarity is positive. Such net charge of a second polarity maygenerally repel free charge carriers of the second polarity towards acollection region on a back surface of a semiconductor substrate. Thismay reduce recombination losses of a radiation sensor element. In otherembodiments, wherein a radiation sensor element comprises dielectricmaterial on high aspect ratio nanostructures, said dielectric materialmay or may not have a net charge, which may or may not be of a secondpolarity. In other embodiments, wherein a radiation sensor elementcomprises, on high aspect ratio nanostructures, dielectric material witha net charge of a second polarity, each pixel portion of a plurality ofpixel portions may or may not comprise a frontal substrate layer,extending along a front surface of a semiconductor substrate and havingmajority charge carriers of a first polarity. Generally, a frontalsubstrate layer having majority charge carriers of a first polarity mayexhibit reduced Auger recombination, which may increase a sensitivity ofa radiation sensor element, especially at shorter wavelengths.

The dielectric material 150 of the embodiment of FIGS. 1 and 2 maycomprise, for example, positively charged silicon oxide(s), such asnon-stoichiometric silicon oxide (SiO_(x)). In other embodiments, anysuitable dielectric material(s), such as SiOx or negatively chargedaluminum oxide(s), e.g., non-stoichiometric aluminum oxide (AlOx), maybe used.

In the embodiment of FIGS. 1 and 2 , the radiation sensor element 100may comprise a scintillator 170. Such scintillator 170 may be arranged,for example, on the front side of the semiconductor substrate 101, asdepicted in FIG. 1 using dashed lines. Generally, a scintillator mayenable converting ionizing radiation to nonionizing electromagneticradiation detectable using a radiation sensor element with asemiconductor substrate, which exhibits weaker interactions withhigh-energy radiation. In other embodiments, a radiation sensor elementmay or may not comprise such scintillator. In embodiments, wherein aradiation sensor element comprises a scintillator on a front side of asemiconductor substrate, said scintillator may or may not be coupled tosaid semiconductor substrate directly or indirectly.

In the embodiment of FIGS. 1 and 2 , the radiation sensor element 100comprises multiple intermediate portions 140 between individual pixelportions of the plurality of pixel portions 110. In FIG. 1 , one suchintermediate portion 140 is highlighted using dashed lines. In FIG. 2 ,a different intermediate portion 140, arranged between the two exemplarypixel portions 111, 112 is depicted. In other embodiments, a radiationsensor element may or may not comprise at least one intermediateportion, arranged between individual pixel portions of a plurality ofpixel portions. In an embodiment, individual pixel portions of aplurality of pixel portions abut one another such that textured regionsof said individual pixel portions form a continuous region on a frontsurface of a semiconductor substrate.

Throughout this disclosure, an “intermediate portion” may refer to apart of a radiation sensor element, which may be arranged between afirst pixel portion and a second pixel portion of a plurality of pixelportions. An intermediate portion may or may not extend from a frontsurface of a semiconductor substrate to a back surface of saidsemiconductor substrate.

Although the intermediate portion 140 highlighted in FIG. 1 is arrangedbetween pixel portions of a single row or column of pixel portions, anintermediate portion may generally exist between any two pixel portionsof a plurality of pixel portions, such as two pixel portions of a singlerow or column of pixel portions and/or two pixel portions arrangeddiagonally with respect to one another.

In the following, the two exemplary pixel portions 111, 112 arediscussed in detail. Although the following discussion is primarilyrelated to the two exemplary pixel portions 111, 112 and theintermediate portion 140 between them, any features disclosed in thefollowing may or may not be applicable to any two pixel portions of theplurality of pixel portions 110 of the embodiment of FIGS. 1 and 2and/or an intermediate portion 140 between said two pixel portions.

The intermediate portion 140 depicted in FIG. 2 comprises anintermediate region 141 on the front surface 102 of the semiconductorsubstrate 101. In other embodiments, an intermediate portion between anytwo pixel portions of a plurality of pixel portions may or may notcomprise such intermediate region. In an embodiment, wherein a radiationsensor element comprises a scintillator arranged on a front side of saidradiation sensor element, said scintillator is coupled to anintermediate region of an intermediate portion. In said embodiment, saidcoupling may be direct or indirect. As such, said coupling may beeffected by any suitable means, for example, at least partly by a gluingprocess, which may or may not comprise an underfilling step.

Herein, an “intermediate region” may refer to a region on a frontsurface of a semiconductor substrate.

Additionally or alternatively, an intermediate region may be arrangedlaterally between two pixel portions of a plurality of pixel portions.

The intermediate region 141 depicted in FIG. 2 has a root mean square(RMS) roughness lower than RMS roughnesses of the textured regions 130of the two exemplary pixel portions 111, 112. Such lower RMS roughnessmay result in a higher reflectance for light incident on an intermediateportion from a front side of a radiation sensor element over a widespectral range. This may reduce generation of free charge carriers in anintermediate portion and/or a vicinity thereof. Such reduction ingeneration of free charge carriers may reduce cross talk betweenindividual pixels of a radiation sensor element, especially in theabsence of a scintillator. In other embodiments, a radiation sensorelement may or may not comprise at least one intermediate portionbetween any two pixel portions of a plurality of pixel portions, theintermediate portion comprising an intermediate region on a frontsurface with a RMS roughness lower than a RMS roughness of a texturedregion of either of the two pixel portions.

In this disclosure, “roughness” may refer to surface roughness withhigher spatial frequencies.

Specifically, “root mean square roughness” may refer to a quadratic meanroughness, which may be measured for a surface and/or a cross-sectionalprofile thereof. Such measurement may be conducted according to at leastone of standards ISO 4287:1997 and ISO 25178. In such measurements, anysuitable method(s), such as atomic force microscopy (AFM), scanningelectron microscopy (SEM), scanning tunneling microscopy (STM), and/orprofilometry, may be used. In case of a textured region, suchmeasurements may be conducted for a macroscopic part of said texturedregion.

In this specification, “macroscopic part” may refer to at least part ofa region, which is sufficiently large to be visible to the naked eye.Additionally or alternatively, a macroscopic part of a textured regionmay encompass or extend over at least 100, or at least 400, or at least900 texture units, such as high aspect ratio nanostructures.

The RMS roughness of the intermediate region 141 depicted in FIG. 2 maybe less than or equal to 0.2 times a RMS toughness of a textured regionof either of the two pixel portions 111, 112. As such, the RMS roughnessof the intermediate region 141 may be higher than or equal to five timesa RMS roughness of the textured region 130 of the first pixel portion111 and higher than or equal to five times a RMS roughness of thetextured region 130 of the second pixel portion 112. Generally, a higherRMS roughness difference may result in lower cross talk betweenindividual pixels of a radiation sensor element. In other embodiments,any suitable ratio may be used between RMS roughnesses of two texturedregions and a RMS roughness of an intermediate region arranged betweensaid two textured regions. For example, a RMS roughness of anintermediate region between two pixel portions may be less than or equalto 0.2, or 0.1, or 0.05, or 0.01 times a RMS roughness of a texturedregion of either of said two pixel portions.

The intermediate region 141 depicted in FIG. 2 extends substantiallyparallel to the base plane 104. An intermediate region extendingsubstantially parallel to a base plane may generally facilitate couplinga scintillator to a semiconductor substrate. In other embodiments, anintermediate region may or may not extend substantially parallel to abase plane.

The textured regions 130 of the two pixel portions 111, 112 depicted inFIG. 2 have a maximum distance d_(tr) ^(max) to the base plane 104.Additionally, the intermediate region 141 depicted in FIG. 2 has aminimum distance d_(ir) ^(min) to the base plane 104 greater than d_(tr)^(max). Such d_(ir) ^(min) may generally further facilitate coupling ascintillator to a semiconductor substrate. This may be due to reducedrisk of high aspect ratio nanostructures breaking during said coupling,since direct contact between the high aspect ratio nanostructures andthe scintillator may be avoided. In other embodiments, a d_(ir) ^(min)may or may not be greater than d_(tr) ^(max) for any intermediateportion between any two pixel portions of a plurality of pixel portions.

With reference to FIG. 2 , the radiation sensor element 100 comprises ametal coating 160. The metal coating 160 is arranged on the front sideof the semiconductor substrate 101 such that a projection of the metalcoating 160 on the base plane 104 intersects a projection of theintermediate region 141 on the base plane 104. Such metal coating mayincrease a reflectivity of a radiation sensor element for light incidenttowards an intermediate portion thereof over a wide spectral range,which may reduce generation of free charge carriers in an intermediateportion and/or a vicinity thereof. Such reduction in generation of freecharge carriers may reduce cross talk between individual pixels of aradiation sensor element, especially in the absence of a scintillator.In other embodiments, a radiation sensor element may or may not comprisesuch metal coating.

Herein, a “coating” may refer to an element, such as a layer, extendingover a surface or part thereof. Specifically, a coating “on a frontside” of a region may or may not be directly or indirectly coupled tosaid region. Further, a “metal coating” may refer to a coating, whichcomprises or is formed of metallic material.

It is to be understood that any of the preceding embodiments of thefirst aspect may be used in combination with each other. In other words,several of the embodiments may be combined together to form a furtherembodiment of the first aspect.

Above, mainly structural and material aspects of radiation sensorelements are discussed. In the following, more emphasis will lie onaspects related to methods for fabricating a radiation sensor elementcomprising a plurality of pixel portions. What is said above about theways of implementation, definitions, details, and advantages related tothe structural and material aspects apply, mutatis mutandis, to themethod aspects discussed below. The same applies vice versa.

It is specifically to be understood that a method according to thesecond aspect may be used to provide a radiation sensor elementaccording to the first aspect and any of the embodiments described inrelation to the first aspect. Correspondingly, any radiation sensorelement according to any embodiment of the first aspect may befabricated using a method according to the second aspect.

FIG. 3 illustrates a method 300 for fabricating a radiation sensorelement comprising a plurality of pixel portions. In other embodiments,such method may comprise processes and/or steps similar or substantiallydifferent to the processes and/or steps, respectively, of the embodimentof FIG. 3 .

In this specification, a “process” may refer to a series of one or moresteps, leading to an end result. Additionally, a “step” may refer to ameasure taken in order to achieve one or more pre-defined end results.Generally, a process may be a single-step or a multi-step process.Additionally, a process may be divisible to a plurality ofsub-processes, wherein individual sub-processes of such plurality ofsub-processes may or may not share common steps.

In the embodiment of FIG. 3 , the method 300 comprises, in process 301,providing a semiconductor substrate. The semiconductor substrate hasbulk majority charge carriers of a first polarity, which may be eithernegative or positive. The semiconductor substrate also has a bulkrefractive index and a front surface, which defines a front side of thesemiconductor substrate. The semiconductor substrate further has a backsurface, arranged opposite the front surface and extending substantiallyalong a base plane.

The method 300 of the embodiment of FIG. 3 comprises, in process 302,forming a collection region on the back surface for each pixel portionof the plurality of pixel portions. The collection regions are suitablefor collecting free charge carriers of a second polarity opposite insign to the first polarity. In some embodiments, a process of forming acollection region may comprise, for example, an ion implantation step.

In the embodiment of FIG. 3 , the method 300 comprises, in process 303,forming a textured region on the front surface for each pixel portion ofthe plurality of pixel portions. Each textured region comprises highaspect ratio nanostructures, extending substantially along a thicknessdirection perpendicular to the base plane and forming an opticalconversion layer, having an effective refractive index graduallychanging towards the bulk refractive index of the semiconductorsubstrate. This reduces reflection of light incident on said pixelportion from the front side of the semiconductor substrate.

In the embodiment of FIG. 3 , the process 303 of forming a texturedregion may comprise, for example, a metal-assisted chemical etching(MACE) step, an atmospheric dry etching (ADE) step, and/or a reactiveion etching (RIE) step, such as a deep reactive ion etching (DRIE) step.Generally, such etching steps may facilitate forming textured regions ina manner resulting in a low defect density and well-defined nanoscalefeatures. In other embodiments, a process of forming a textured regionmay comprise any suitable step(s), for example, a MACE step, an ADEstep, and/or a RIE step, such as a DRIE step.

In the embodiment of FIG. 3 , the method 300 comprises, in optionalprocess 304, forming an intermediate portion between two pixel portionsof the plurality of pixel portions. The intermediate portion comprisesan intermediate region on the front surface of the semiconductorsubstrate with a RMS roughness lower than a RMS roughness of a texturedregion of either of the two pixel portions. In other embodiments, amethod for fabricating a radiation sensor element comprising a pluralityof pixel portions may or may not comprise such process of forming anintermediate portion.

As indicated by the ordering of boxes representing processes 302, 303,and 304, steps implementing said processes 302, 303, 304 may generallybe completed in various alternative orders. Further, a step maygenerally contribute to more than one of said processes 302, 303, 304.For instance, an etching step, e.g., a MACE step, an ADE step, and/or aRIE step, such as a DRIE step, may contribute both to a process offorming a textured region and to a process of forming an intermediateportion. Such may occur, for example, if a semiconductor substratehaving a polished front surface is provided and etching in said etchingstep is done locally, for example, using an etching mask.

In the embodiment of FIG. 3 , the method 300 comprises, in optionalprocess 305, coupling a scintillator onto the intermediate region. Suchprocess may generally comprise, for example, an adhesive bonding step.In other embodiments, a method for fabricating a radiation sensorelement comprising a plurality of pixel portions may or may not comprisesuch process of coupling a scintillator.

In an embodiment, a method for fabricating a radiation sensor elementcomprising a plurality of pixel portions comprises processescorresponding to the processes 301, 302, and 303 of the method 300 ofthe embodiment of FIG. 3 . In other embodiments, a method forfabricating a radiation sensor element comprising a plurality of pixelportions may comprise processes corresponding to the processes 301, 302,and 303 as well as at least one of the processes 304 and 305 of themethod 300 of the embodiment of FIG. 3 .

Generally, steps of a method for fabricating a radiation sensor elementcomprising a plurality of pixel portions implementing processescorresponding to any of the processes 301, 302, 303, 304, 305 of themethod 300 of the embodiment of FIG. 3 may be executed in multiplealternative orders. Additionally, a method for fabricating a radiationsensor element comprising a plurality of pixel portions may generallycomprise any number of additional processes or steps that are notdisclosed herein in connection to the method 300 of the embodiment ofFIG. 3 .

For example, in an embodiment, a method for fabricating a radiationsensor element, comprising a plurality of pixel portions, comprisesdepositing dielectric material onto high aspect ratio nanostructures,the dielectric material conformally coating the high aspect rationanostructures.

In another embodiment, which may be in accordance with the precedingembodiment, a method for fabricating a radiation sensor element,comprising a plurality of pixel portions, comprises forming a metalcoating on a front side of an intermediate region. In said embodiment,the process of forming a metal coating may comprise, for example, anevaporation step or a sputtering step.

It is obvious to a person skilled in the art that with the advancementof technology, the basic idea of the invention may be implemented invarious ways. The invention and its embodiments are thus not limited tothe examples described above, instead they may vary within the scope ofthe claims.

It will be understood that any benefits and advantages described abovemay relate to one embodiment or may relate to several embodiments. Theembodiments are not limited to those that solve any or all of the statedproblems or those that have any or all of the stated benefits andadvantages.

The term “comprising” is used in this specification to mean includingthe feature(s) or act(s) followed thereafter, without excluding thepresence of one or more additional features or acts. It will further beunderstood that reference to ‘an’ item refers to one or more of thoseitems.

REFERENCE SIGNS

-   -   100 radiation sensor element    -   101 semiconductor substrate    -   102 front surface    -   103 back surface    -   104 base plane    -   110 plurality of pixel portions    -   111 pixel portion    -   112 pixel portion    -   120 collection region    -   121 collection doping well    -   122 bulk contact region    -   123 bulk contact doping well    -   130 textured region    -   135 high aspect ratio nanostructures    -   136 optical conversion layer    -   140 intermediate portion    -   141 intermediate region    -   150 dielectric material    -   160 metal coating    -   170 scintillator    -   300 method    -   301 providing a semiconductor substrate    -   302 forming a collection region    -   303 forming a textured region    -   304 forming an intermediate portion    -   305 coupling a scintillator

1. A radiation sensor element comprising a semiconductor substrate,having bulk majority charge carriers of a first polarity, a bulkrefractive index, a front surface, defining a front side of thesemiconductor substrate, and a back surface, arranged opposite the frontsurface and extending substantially along a base plane; the radiationsensor element comprising a plurality of pixel portions, each pixelportion of the plurality of pixel portions comprising a collectionregion on the back surface for collecting free charge carriers of asecond polarity opposite in sign to the first polarity; wherein eachpixel portion of the plurality of pixel portions comprises a texturedregion on the front surface, the textured region comprising high aspectratio nanostructures, extending substantially along a thicknessdirection perpendicular to the base plane and forming an opticalconversion layer, having an effective refractive index graduallychanging towards the bulk refractive index to reduce reflection of lightincident on said pixel portion from the front side of the semiconductorsubstrate; the radiation sensor element comprises an intermediateportion between two pixel portions of the plurality of pixel portions,the intermediate portion comprising an intermediate region on the frontsurface with a root mean square, RMS, roughness lower than a RMSroughness of a textured region of either of the two pixel portions.
 2. Aradiation sensor element according to claim 1, wherein the RMS roughnessof the intermediate region is less than or equal to 0.2 times a RMSroughness of a textured region of either of the two pixel portions.
 3. Aradiation sensor element according to claim 1, wherein the intermediateregion extends substantially parallel to the base plane.
 4. A radiationsensor element according to claim 1, comprising a metal coating on thefront side of the semiconductor substrate, a projection of the metalcoating on the base plane intersecting a projection of the intermediateregion on the base plane.
 5. A radiation sensor element according toclaim 1, wherein the semiconductor substrate and the high aspect rationanostructures are formed of silicon, Si.
 6. A radiation sensor elementaccording to claim 1, wherein the radiation sensor element comprisesdielectric material conformally coating the high aspect rationanostructures.
 7. A radiation sensor element according to claim 6,wherein the dielectric material has a net charge of the second polarity.8. A radiation sensor element according to claim 1, wherein thecollection region is defined by a collection doping well with majoritycharge carriers of the second polarity.
 9. A method for fabricating aradiation sensor element comprising a plurality of pixel portions, themethod comprising: providing a semiconductor substrate, having bulkmajority charge carriers of a first polarity, a bulk refractive index, afront surface, defining a front side of the semiconductor substrate, anda back surface, arranged opposite the front surface and extendingsubstantially along a base plane; for each pixel portion of theplurality of pixel portions, forming a collection region on the backsurface for collecting free charge carriers of a second polarityopposite in sign to the first polarity; for each pixel portion of theplurality of pixel portions, forming a textured region (303) on thefront surface, the textured region comprising high aspect rationanostructures, extending substantially along a thickness directionperpendicular to the base plane and forming an optical conversion layer,having an effective refractive index gradually changing towards the bulkrefractive index to reduce reflection of light incident on said pixelportion from the front side of the semiconductor substrate; wherein anintermediate portion is formed between two pixel portions of theplurality of pixel portions, the intermediate portion comprising anintermediate region on the front surface with a root mean square, RMS,roughness lower than a RMS roughness of a textured region of either ofthe two pixel portions.
 10. A method according to claim 9, wherein theprocess of forming a textured region comprises a metal-assisted chemicaletching, MACE, step, an atmospheric dry etching, ADE, step, and/or areactive ion etching, RIE, step.
 11. A method according to claim 9,wherein the RMS roughness of the intermediate region is less than orequal to 0.2 times a RMS roughness of a textured region of either of thetwo pixel portions.
 12. A radiation sensor element according to claim 1,wherein the RMS roughness of the intermediate region is less than orequal to 0.1 times a RMS roughness of a textured region of either of thetwo pixel portions.
 13. A radiation sensor element according to claim 1,wherein the RMS roughness of the intermediate region is less than orequal to 0.05 times a RMS roughness of a textured region of either ofthe two pixel portions.
 14. A radiation sensor element according toclaim 1, wherein the RMS roughness of the intermediate region is lessthan or equal to 0.01 times a RMS roughness of a textured region ofeither of the two pixel portions.
 15. A method according to claim 9,wherein the process of forming a textured region comprises ametal-assisted chemical etching, MACE, step, an atmospheric dry etching,ADE, step, and/or a deep reactive ion etching, DRIE, step.
 16. A methodaccording to claim 9, wherein the RMS roughness of the intermediateregion is less than or equal to 0.1 times a RMS roughness of a texturedregion of either of the two pixel portions.
 17. A method according toclaim 9, wherein the RMS roughness of the intermediate region is lessthan or equal to 0.05 times a RMS roughness of a textured region ofeither of the two pixel portions.
 18. A method according to claim 9,wherein the RMS roughness of the intermediate region is less than orequal to 0.01 times a RMS roughness of a textured region of either ofthe two pixel portions.